clarify codec_tag

Originally committed as revision 8484 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -971,6 +971,13 @@ typedef struct AVCodecContext {
     /**
      * fourcc (LSB first, so "ABCD" -> ('D'<<24) + ('C'<<16) + ('B'<<8) + 'A').
      * this is used to workaround some encoder bugs
+     * a demuxer should set this to what is stored in the field used to identify the codec
+     * if there are mutiple such fields in a container then the demuxer should choose the one
+     * which maximizes the information about the used codec
+     * if the codec tag field in a container is larger then 32bit then the demxuer should
+     * remap the longer id to 32bit with a table or other structure alternatively a new
+     * extra_codec_tag + size could be added but for this a clear advantage must be demonstrated
+     * first
      * - encoding: set by user, if not then the default based on codec_id will be used
      * - decoding: set by user, will be converted to upper case by lavc during init
      */